The isolated perfused kidney of the rat will be used as a model for the investigation of factors regulating renal function. The regional distribution of blood flow will be studied by the use of radioactive microspheres and the possible contribution of changes in flow to hormonal effects on the perfused kidney will be measured. The action of parathyroid hormone on tubular function will be studied in this model. Metabolic events correlated with changes in sodium reabsorption will also be studied by analyzing kidney tissue and kidney perfusate for key metabolites. The active transport of chloride will be studied in isolated tubules and cells from a variety of tissues and its dependence upon concomitant sodium transport and the activity of Na-K-ATPase investigated. The comparative physiology of ion transport across gills will be studied, with special reference to hormonal adaptations during transfer from seawater to fresh water. BIBLIOGRAPHIC REFERENCES: Silva, P., Torretti, J., Hayslett, J.P. and Epstein, F.H. Relation between Na-K-ATPase activity and respiratory rate in the rat kidney. Am. J. Physiol. 230, 1432-1438, 1976. Besarab, A., Silva, P. and Epstein, F.H. Multiple pumps for sodium reabsorption by the perfused kidney. Kidney Int. 10, 27-33, 1976.